Nitration reactions of aromatic hydrocarbons are generally conducted in mixed acid systems, such as mixed nitric and sulfuric acids. However, these mixed acid systems usually involve reconcentration of the spent sulfuric acid after the nitration reaction. This reconcentration step is time consuming, energy intensive and requires the use of expensive materials of construction. In addition, the use of sulfuric acid tends to result in significant nitrocreosol and cyanide by-product formation which requires expensive waste-water treatment to remove.
In view of these disadvantages associated with mixed nitric/sulfuric acid systems, there have been recent attempts to perform gas phase or liquid phase nitrations in concentrated nitric acid in the absence of sulfuric acid. By way of illustration, U.S. Pat. No. 4,064,147 discloses the preparation of aromatic mononitro compounds (such as mononitrobenzene) by a liquid phase reaction with nitric acid having an acid concentration of between 70 percent and 100 percent by weight using a reaction temperature of between 0.degree. C. and 80.degree. C. When employing a relatively reactive compound such as benzene or toluene as a starting material, this patent teaches that a nitric acid concentration of between 70 and 90 percent by weight is preferred. The disclosure of this patent requires a ratio of nitric acid plus water to organic components of not below 3 when using 70 percent nitric acid, and not below 8 when using 100 percent nitric acid. However, it has now been found that such a high acid ratio using 100 percent nitric acid tends to favor dinitro-compound production, not desired by the patentee in the '147 patent.
U.S. Pat. No. 2,362,743 discloses a two-step process for the manufacture of dinitrotoluene ("DNT") in the absence of sulfuric acid which comprises (a) nitrating toluene to mononitrotoluene using a nitric acid having a concentration of from about 60 percent to about 75 percent and a mole ratio of toluene to nitric acid of about 1 to about 3.5 and (b) nitrating the mononitrotoluene to dinitrotoluene using nitric acid having a concentration of from about 90 percent to about 100 percent, and a mole ratio of mononitrotoluene to nitric acid of about 1 to about 3. Although the process of this '743 patent is advantageously conducted in the absence of sulfuric acid, it has now been found that in step (b), a very high percentage of the nitrated product (up to 25 percent) based upon the amount of toluene reactant employed does not phase separate from the nitric acid medium. Indeed, the '743 patent teaches vacuum distillation of the product mixture to isolate the desired dinitrotoluene, an expensive and highly energy intensive process step.
U.S. Pat. No. 3,928,395 discloses a process for nitrating unsubstituted or substituted benzene at a reaction temperature of -40.degree. C. to 80.degree. C. using 90% to 100% nitric acid in the optional and preferred presence of a dipolar aprotic solvent, wherein the reaction is halted by means of a dipolar aprotic solvent.
Since dinitrotoluene is useful as an intermediate in producing TDI, new processes for the selective manufacture of this intermediate would be highly desirable to the polyisocyanate manufacturing community.